[RFC PATCH v2 09/25] include/block/blockjob_int.h: split header into I/O and GS API

[Emanuele Giuseppe Esposito]

Since the I/O functions are not many, keep a single file.
Also split the function pointers in BlockJobDriver.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 include/block/blockjob_int.h | 55 ++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/include/block/blockjob_int.h b/include/block/blockjob_int.h
index 6633d83da2..bac4e8f46d 100644
--- a/include/block/blockjob_int.h
+++ b/include/block/blockjob_int.h
@@ -38,6 +38,12 @@ struct BlockJobDriver {
     /** Generic JobDriver callbacks and settings */
     JobDriver job_driver;
 
+    /*
+     * I/O API functions. These functions are thread-safe, and therefore
+     * can run in any thread as long as they have called
+     * aio_context_acquire/release().
+     */
+
     /*
      * Returns whether the job has pending requests for the child or will
      * submit new requests before the next pause point. This callback is polled
@@ -46,6 +52,28 @@ struct BlockJobDriver {
      */
     bool (*drained_poll)(BlockJob *job);
 
+    /*
+     * Global state (GS) API. These functions run under the BQL lock.
+     *
+     * If a function modifies the graph, it also uses drain and/or
+     * aio_context_acquire/release to be sure it has unique access.
+     * aio_context locking is needed together with BQL because of
+     * the thread-safe I/O API that concurrently runs and accesses
+     * the graph without the BQL.
+     *
+     * It is important to note that not all of these functions are
+     * necessarily limited to running under the BQL, but they would
+     * require additional auditing and may small thread-safety changes
+     * to move them into the I/O API. Often it's not worth doing that
+     * work since the APIs are only used with the BQL held at the
+     * moment, so they have been placed in the GS API (for now).
+     *
+     * All callers that use these function pointers must
+     * use this assertion:
+     * g_assert(qemu_in_main_thread());
+     * to catch when they are accidentally called without the BQL.
+     */
+
     /*
      * If the callback is not NULL, it will be invoked before the job is
      * resumed in a new AioContext.  This is the place to move any resources
@@ -56,6 +84,27 @@ struct BlockJobDriver {
     void (*set_speed)(BlockJob *job, int64_t speed);
 };
 
+/*
+ * Global state (GS) API. These functions run under the BQL lock.
+ *
+ * If a function modifies the graph, it also uses drain and/or
+ * aio_context_acquire/release to be sure it has unique access.
+ * aio_context locking is needed together with BQL because of
+ * the thread-safe I/O API that concurrently runs and accesses
+ * the graph without the BQL.
+ *
+ * It is important to note that not all of these functions are
+ * necessarily limited to running under the BQL, but they would
+ * require additional auditing and may small thread-safety changes
+ * to move them into the I/O API. Often it's not worth doing that
+ * work since the APIs are only used with the BQL held at the
+ * moment, so they have been placed in the GS API (for now).
+ *
+ * All functions below must use this assertion:
+ * g_assert(qemu_in_main_thread());
+ * to catch when they are accidentally called without the BQL.
+ */
+
 /**
  * block_job_create:
  * @job_id: The id of the newly-created job, or %NULL to have one
@@ -98,6 +147,12 @@ void block_job_free(Job *job);
  */
 void block_job_user_resume(Job *job);
 
+/*
+ * I/O API functions. These functions are thread-safe, and therefore
+ * can run in any thread as long as they have called
+ * aio_context_acquire/release().
+ */
+
 /**
  * block_job_ratelimit_get_delay:
  *
-- 
2.27.0